Solve for $y$ : $-3 = -18 + y$
Add $18$ to both sides: $ \begin{eqnarray} \\ \hphantom{-3 {+ 18}} &\hphantom{=}& \hphantom{y} \\ -3 &=& -18 + y \\ \\ {+18} && {+18} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -3 {+ 18} &=& y \\ \end{eqnarray} $ Simplify. $ y = 15$